TOKYO -- At least five frogs have died in Japan's first confirmed cases of a fungal infection linked to sharp reductions in amphibian numbers in other parts of the world, an expert said Friday.

The discovery prompted animal and research groups in Japan to jointly declare "a state of emergency," urging frog owners to contact veterinarians immediately for any abnormalities.

Yumi Une, assistant professor of Azabu University in Kanagawa, just west of Tokyo, said that at least five frogs tested positive for the chytrid fungus recently.

Two of the five dead frogs were kept as pets by a couple in Tokyo and tested positive for the fungus in late December while the infection of three others kept in a pet shop near Tokyo was confirmed earlier this month, according to Une.

The dead frogs were of South American origin and are believed to have been raised in Japan, she said.

A dozen other frogs owned by the couple had died earlier, and their deaths may have been caused by the same deadly fungus, Une said. The frogs were kept in several water tanks inside the couple's house and the fungus was possibly transmitted through water, she said.

It is the first time that the fungus has been confirmed in frogs in Japan, according to Une. In Asia, only Australia had confirmed cases of the fungus infection.

"Although these were the first confirmed cases, there is no proof that there had been no infection before," Une said. She said there had been no reports of massive deaths of wild frogs, a situation more grave because of the difficulties to contain infection.

The emergency declaration, posted on the Web site of World Wide Fund for Nature Japan Friday and dated Saturday, urged owners of frogs and other amphibious animals to be more vigil and authorities to strengthen quarantine.

The parasitic skin fungus has a more than 90 percent likelihood of killing an amphibian, but is harmless to other species including human beings.

The chytrid fungus kills the frogs by growing on their skin, making it hard for them to use their pores and regulate water intake. The frogs die of dehydration in the water.

Frogs and many other amphibians are acutely sensitive to changes in environmental temperature and humidity as they can not maintain a steady internal temperature to the same extent as birds and mammals.

It is believed to be a major cause of the dramatic reduction of the number of amphibians in many parts of the world.